1380 Part VIII / Learning, Memory, and Cognition
Figure 55–4 Dual-stream model of language
processing. Temporal and spectral analyses of
speech signals occur bilaterally in the auditory
cortex followed by phonological analysis in the
posterior superior temporal gyri (yellow arrow).
Processing then diverges into two separate
pathways: a dorsal stream that maps speech
sounds to motor programs and a ventral stream
that maps speech sounds to meaning. The dorsal
pathway is strongly left hemisphere dominant
and has segments that extend to the premotor
cortex (dorsal pathway 1) and to the posterior
inferior frontal cortex (dorsal pathway 2). The
ventral pathway occurs bilaterally and extends
to the anterior temporal lobe and the posterior
inferior frontal cortex. (Adapted, with permission,
from Hickok and Poeppel 2007, and Skeide and
Friederici 2016.)
superior temporal gyrus to the premotor cortex, and
dorsal pathway 2 connects the posterior superior tem-
poral gyrus to Broca’s area. Pathway 2 is involved in
higher-order analysis of speech, such as discriminat-
ing subtle differences in meaning based on grammar
and interpreting language using more complex con-
cepts. The dorsal stream is strongly left hemisphere
dominant. The arcuate fasciculus and the superior lon-
gitudinal fasciculus are white matter fiber tracts that
mediate communication along the dorsal stream.
The ventral stream passes below the Sylvian fissure
and is composed of regions of the superior and mid-
dle temporal lobes as well as regions of the posterior
inferior frontal lobe. This stream conveys information
for auditory comprehension, which requires transfor-
mation of the auditory signal to representations in a
mental lexicon, a “brain-based dictionary” that links
individual word forms to their semantic meaning. This
stream comprises the inferior fronto-occipital fascicu-
lus, the uncinate fasciculus, and the extreme fiber cap-
sule system and is largely bilaterally represented.
The cortical brain regions included in the dual-
stream model also interact with spatially distributed
regions throughout both hemispheres of the brain that
provide additional information crucial for language
processing. These regions include the prefrontal cortex
and cingulate cortices, which exert executive control and
mediate attentional processes, respectively, as well as
regions in the medial temporal, frontal, and parietal
areas involved in memory retrieval.
The Neural Architecture for Language Develops
Rapidly During Infancy
The study of language development in infancy requires
a methodology that documents significant changes in
behavior and links those changes to changes in brain
function and morphology over time. Neuroimaging
methods for the infant brain have improved substan-
tially over the past decade, allowing for a detailed
assessment of the progression of development of
the specialized regions and structural connections
required by the language network. For example, devel-
opmental neuroscientists have created models of the
average infant brain and brain atlases for the infant
brain at 3 and 6 months of age. These models indicate
that brain structures essential to language processing
in adulthood, such as the inferior frontal cortex, pre-
motor cortex, and superior temporal gyrus, support
speech processing in early infancy. Studies using DTI
and tractography indicate that the arcuate fasciculus
and the uncinate fasciculus connect language regions
by 3 months of age.
The development of the neural substrates for lan-
guage in 1- to 3-day-old infants has been studied in
depth by Daniela Perani using fMRI and DTI. Perani’s
fMRI work reveals that listening to speech activates
the infant superior temporal gyrus bilaterally and
that in the left hemisphere this activation extends to
the planum temporale, inferior frontal gyrus, and
inferior parietal lobe. Perani’s DTI studies of the same
newborn infants demonstrate weak intrahemispheric
connections, but strong connections between the hemi-
spheres. Nevertheless, the ventral fiber tract connect-
ing the ventral portion of the inferior frontal gyrus via
the extreme fiber capsule system to the temporal cortex
is evident in newborns and in both hemispheres. The
dorsal pathway connecting the temporal cortex to
the premotor cortex is also present in the newborns,
although the dorsal tract that connects the temporal
cortex to Broca’s area in adults is not detectable in new-
borns. These early connections between sensory areas
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